When glTF export in Blender2.8, an animation is not played well
This should be fixed in the next version of Blender's glTF exporter, where the default value for Always sample animations has been changed to true (in #706).  I believe putting a checkmark on this option may be a quick fix for your problem.

Animations involving rotations are stored in glTF using quaternions (because glTF is a runtime delivery format, meant for rapid load-and-render operations with minimal pre-processing by the loader).  But quaternions have an important limitation, they can't distinguish between +180 and -180 degrees.  If you have Euler angle keyframes at 0 and 360 degrees, both of those values convert to the same identical quaternion, because the object is in the same orientation.  Interpolation between keys breaks down.

The usual solution is to place intermediate keyframes at 90 degree increments (or no larger than 120 degree increments).  One way to do this is just check Always sample animations which makes every frame a keyframe.